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The prevalence of pathogenic Yersinia enterocolitica in pig herds was monitored during six trials (at four different farrow-to-finisher farms). Samples were taken throughout the whole rearing period from birth of the piglets to the final fattening stage, and different samples were taken from these pigs during the slaughter process. Environmental samples also were evaluated to identify potential sources of on-farm infection. Y. enterocolitica was isolated using irgasan-ticarcillin-potassium chlorate broth enrichment and cefsulodin-irgasan-novobiocin agar culture. Colonies were identified using bio- and serotyping methods and by PCR assay. Pathogenic Y. enterocolitica were not isolated from fecal samples from piglets and weaners. The only fecal samples positive for Y. enterocolitica were obtained during the fattening stage. The prevalence of Y. enterocolitica in fattening pig herds ranged between 0 and 65.4%. Y. enterocolitica isolates were detected at the abattoir in 38.4% of the tonsils, in 3.8% of the ileocecal lymph nodes, on 0.3% of the carcass surfaces before chilling, and on 0% of the carcass surfaces after chilling. Almost all isolates belonged to bioserotype 4/O:3. Only one strain was identified as O:9. All isolates contained the ail gene. The yopT gene was found in 99.1% of the farm isolates but in only 76.6% of the isolates found at the abattoir from the corresponding carcasses. Although a direct link between porcine isolates and human infection has not been demonstrated, the similarity of the bioserotypes in infected pigs and humans and the presence of virulence factors in porcine isolates should encourage further studies to determine the risk of transmission of Y. enterocolitica to humans from pigs and pork products.